It is highly likely that portable electronic apparatuses and the like are dropped by mistake because of characteristics thereof. Portable electronic apparatuses such as a notebook personal computer and a part of digital music players having magnetic disks built therein are particularly delicate to shocks and require protection measures against fall. Such protection measures are described in Patent Document 1 and Patent Document 2. In Patent Document 1, a three-axis acceleration sensor is provided and, when acceleration signals of all three axes indicate small acceleration of substantially zero and the acceleration continues for a certain time, it is judged that a portable electronic apparatus is in free falling, and a magnetic head is moved to a retraction area to prevent breakage of the magnetic disk due to shock at the time of fall and collision. In Patent Document 2, when a magnitude of a composite vector of three-axis accelerations exceeds a certain threshold for 90 milliseconds or more, fall is detected and a magnetic head is moved to a retraction area to prevent breakage of the magnetic disk due to shock at the time of fall and collision.
Patent Document 1: Japanese Patent Laid-Open No. 2000-241442
Patent Document 2: U.S. Pat. No. 5,982,573